The present invention relates to a multi-stage compressor, and more specifically, to a multi-stage compressor whose rotor is supported by magnetic bearings.
A compressed gas in a compressor is partially bled so as to cool a motor or magnetic bearings, bleeding as disclosed in Japanese Utility Model Unexamined Publication No. 3-19498. In this prior art, a centrifugal impeller is provided on an end portion of a rotating shaft of the motor, a motor rotor is supported by the magnetic bearings to constitute the compressor which cooperates with a condenser and an evaporator to constitute a refrigerating cycle. In the refrigerating cycle, a part of refrigerant is sprayed from a spray nozzle so as to cool a coil of the motor and the magnetic bearings.
Japanese Patent Unexamined Publication No. 64-80799 discloses a compressor, in which a compressor stage is combined with a high-frequency motor, a rotating shaft is supported by magnetic bearings, and which is covered with a integrated housing sealed against an outside. In this compressor, gas conduits provided between a plurality of compressor stages may function as a surface cooler. That is, a part of the compressed gas re-cooled by the surface cooler is supplied through a supply passage to a rotor of the high-frequency motor, a rotor of an exciter and the magnetic bearings to cool them, and then is returned through a discharge passage to a suction pipe.
When a cooling apparatus described in Japanese Utility Model Unexamined Publication No. 3-19498 is applied to a multi-stage compressor having a high compression ratio, the cooling gas must be partially bled from downstream of a final-stage to accommodate a necessary gas flow at the time of a non-loading operation, resulting in that a cooling air flows more than necessary during a loading operation, which is uneconomical. On the contrary, when the cooling gas is bled from an intermediate stage for the purpose of economy, a negative pressure prevails in an bleeding section during the non-loading operation, resulting in a disadvantage that the cooling gas cannot pass through the motor. In addition, the above-described cooling apparatus is associated with a centrifugal compressor provided in a refrigerating cycle, and premises that a refrigerant is used for cooling the motor or the magnetic bearings, so that an adequate cooling effect cannot be obtained for a compressor for compressing air or industrial gas.
The compressing apparatus described in Japanese Patent Unexamined Publication No. 64-80799 is forced to be a split casing structure, and the casing is surrounded by a coiled surface cooler, so that a large numbers of processes and labor are required for assembly. Moreover, the cooling during the non-loading operation is not considered at all. Furthermore, discharge of drainage from the surface cooler is not taken into consideration, so that when used to compress air or the like containing a water vapor, the drainage may be collected in the surface cooler, or splash of the drainage may be mixed into the cooling gas resulting in condensation within the compressor.